Jeff M. Diebold

University of Illinois at Urbana-Champaign, Urbana, Illinois 

Andy P. Broeren

Glenn Research Center, Cleveland, Ohio

Michael B. Bragg

University of Illinois at Urbana-Champaign, Urbana, Illinois 

Aerodynamic Classification of 

Swept-Wing Ice Accretion

NASA/TM—2013-216556

Sepember 2013

AIAA–2013–2825
NASA STI Program . . . in Profile

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scientific and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Officer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major significant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of significant 

scientific and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scientific 

and technical findings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scientific and 

technical findings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scientific and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scientific, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 443–757–5803

• Phone the NASA STI Information Desk at

443–757–5802

• Write to:

 STI Information Desk

 NASA Center for AeroSpace Information

 7115 Standard Drive

 Hanover, MD 21076–1320
Jeff M. Diebold

University of Illinois at Urbana-Champaign, Urbana, Illinois 

Andy P. Broeren

Glenn Research Center, Cleveland, Ohio

Michael B. Bragg

University of Illinois at Urbana-Champaign, Urbana, Illinois 

Aerodynamic Classification of 

Swept-Wing Ice Accretion

NASA/TM—2013-216556

Sepember 2013

AIAA–2013–2825

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

5th Atmospheric and Space Environments Conference

sponsored by the American Institute of Aeronautics and Astronautics

San Diego, California, June 24–27, 2013
Acknowledgments

The authors would like to thank Joe Botalla at the University of Illinois who wrote the first draft of the literature review of 

swept wings with ice accretion and thus help initiate this paper. Abdi Khodadoust provided hard to acquire references from his 

extensive collection of icing papers. Several colleagues shared their expertise by reading a draft of this document and providing 

helpful comments and suggestions. This includes: Frederic Moens at ONERA; Cris Bosetti, Don Cook, Abdi Khodadoust, Adam 

Malone and Ben Paul at Boeing; Jim Riley at FAA; Sam Lee, Mark Potapczuk, Mary Wadel and Eric Kreeger at NASA Glenn. 

The authors from the University of Illinois were supported in this effort by a grant from the FAA Technical Center, DOT_FAA 

10-G-0004, with technical monitor Jim Riley. NASA’s Atmospheric Environment Safety Technologies Project of the Aviation 

Safety Program also supported this work. 

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identification 

only. Their usage does not constitute an official endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-216556 1

Aerodynamic Classification of Swept-Wing Ice Accretion

Jeff M. Diebold

University of Illinois at Urbana-Champaign

Urbana, Illinois 61801

Andy P. Broeren

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Michael B. Bragg

University of Illinois at Urbana-Champaign

Urbana, Illinois 61801

Abstract

The continued design, certification and safe operation of swept-wing airplanes in icing conditions rely 

on the advancement of computational and experimental simulation methods for higher fidelity results 

over an increasing range of aircraft configurations and performance, and icing conditions. The current 

state-of-the-art in icing aerodynamics is mainly built upon a comprehensive understanding of twodimensional geometries that does not currently exist for fundamentally three-dimensional geometries such 

as swept wings. The purpose of this report is to describe what is known of iced-swept-wing aerodynamics 

and to identify the type of research that is required to improve the current understanding. Following the 

method used in a previous review of iced-airfoil aerodynamics, this report proposes a classification of 

swept-wing ice accretion into four groups based upon unique flowfield attributes. These four groups are: 

ice roughness, horn ice, streamwise ice and spanwise-ridge ice. In the case of horn ice it is shown that a 

further subclassification of “nominally 3-D” or “highly 3-D” horn ice may be necessary. For all of the 

proposed ice-shape classifications, relatively little is known about the three-dimensional flowfield and 

even less about the effect of Reynolds number and Mach number on these flowfields. The classifications 

and supporting data presented in this report can serve as a starting point as new research explores sweptwing aerodynamics with ice shapes. As further results are available, it is expected that these 

classifications will need to be updated and revised.

1.0 Introduction

The formation of ice on lifting surfaces in flight presents a serious risk to aircraft safety as well as a 

multitude of scientific and engineering challenges. Investigations into the accretion of ice and the 

resulting aerodynamic penalties began as early as the 1940s, and the research conducted in the past 

several decades has led to a thorough, but far from complete, understanding of the effects of ice on 

airfoils. Lynch and Khodadoust (Ref. 1) provide a review of the performance effects of ice on airfoils and

straight wings, while Bragg et al. (Ref. 2) reviewed the flowfield characteristics that lead to the observed 

performance effects. In addition to reviewing the flowfield of iced airfoils, Bragg et al. (Ref. 2) proposed 

an ice shape classification system that was based on the unique flowfield features generated by the ice. 

The classifications proved to be very useful when conducting parametric studies of the aerodynamic 

effects of ice on airfoils. 

Our understanding of iced airfoil aerodynamics has reached a level of maturity where we can begin 

trying to understand the complex effects of ice on highly three-dimensional (3-D)swept wings. Swept 

wing icing presents significant challenges because the parameter space for the 3-D wing case is much 

larger and more complex than for airfoils. In addition to airfoil geometry, ice-shape geometry, size and 
NASA/TM—2013-216556 2

location, the 3-D wing geometry must be considered. Here the airfoil and ice accretion can vary along the 

span and wing sweep angle, twist, taper, dihedral and aspect ratio must also be considered. As a result of 

this complexity, there are relatively few studies investigating the effects of ice on swept wing 

aerodynamics. 

NASA, FAA, ONERA, the University of Illinois, Boeing and others are embarking on a research 

program with goals to improve our understanding and ability to model ice accretion and the resulting 

aerodynamic effect on full-scale, 3-D swept wings. The purpose of this review is to provide the initial 

framework for improving our understanding of swept wing icing, by reviewing much of the previous

aerodynamic research in the context of a proposed ice shape classification system, similar to that 

developed by Bragg et al. (Ref. 2) for airfoils, and identify areas where more research is required. The 

classification is based on the fundamental flow physics that are unique to the different ice geometries and 

are responsible for the measured aerodynamic effects. Similar to two-dimensional (2-D) case, the 

proposed classifications are: ice roughness, horn ice, streamwise ice and spanwise-ridge ice. On a swept 

wing, the horn ice accretion can be further classified as with scallops, incomplete scallops or no scallops. 

It will be shown that from an aerodynamic point of view it may be more appropriate to classify the horn 

ice as nominally 3-D and highly 3-D horn ice. This review will discuss each classification with a focus on 

the unique aerodynamic characteristics as they are currently understood. An effort will be made to 

interpret aerodynamic performance in terms of key flowfield features and where appropriate comparisons 

to the 2-D case will be made. It should be noted that experimental swept wing icing data are limited and 

the different ice shape classifications have not received equal attention in the literature. In addition, most

of the swept wing icing experiments have been conducted at low Reynolds numbers, and Reynolds 

number and Mach number effects are unknown. When the ice-shape classifications for airfoils were 

developed there was a better understanding of the aerodynamics and flowfields than currently exists for 3-

D iced wings. Therefore, as research is conducted and the knowledgebase increases, the classifications 

will undoubtedly change and improve. This report is a modified and updated version of the work of 

Broeren, Diebold and Bragg (Ref. 3).

2.0 Aerodynamic Classification of Swept Wing Ice Accretion

The purpose of this section is to review the existing body of technical literature on iced-swept wing 

aerodynamics in the context of a classification method based upon fundamental flowfield characteristics. 

The classifications include roughness, horn ice, streamwise ice and spanwise ridge ice. The focus of the 

following discussion will be on how the ice affects the flowfield and the performance, the physical 

characteristics and mechanisms through which the ice forms will only be discussed briefly. Vargas 

provides an excellent historical and technical summary of swept wing ice accretion (Ref. 4). As 

mentioned above, not all classifications have received an equal amount of attention. Currently, roughness 

and horn ice shapes have been the subject of the most investigations while streamwise and spanwise ridge 

ice have not been studied in as much detail.

2.1 Ice Roughness

Ice roughness refers to surface roughness associated with the initial stages of in-flight ice accretion. 

Anderson and Shin (Ref. 5) characterized roughness formation on airfoils and found that initially the 

leading-edge contains a smooth zone, a rough zone and a feather region. These same features are 

observed for swept wings, but for large sweep angles in both glaze and rime icing conditions it has been 

observed that the smooth zone may not form. Despite this difference, the characteristics of initial icing 

roughness on swept wings are fundamentally the same as for airfoils and straight wings. Icing roughness 

can be characterized by height, location, chordwise extent, concentration and shape (Ref. 2). It has been 

observed on airfoils (Ref. 6) and swept wings (Ref. 4) that the height of the ice roughness elements is 

generally greater than the local boundary layer thickness. Vargas (Ref. 4) points out that the effects of 

roughness of this size on the boundary layer instabilities and transition are unknown. It is reasonable to 
NASA/TM—2013-216556 3

assume that the local separated flowfield in the vicinity of the roughness elements is not fundamentally 

different than in the airfoil case. For this, Bragg et al. (Ref. 2) provide a detailed review of the roughness 

flowfield including the relevant phenomenological features, effects on transition and turbulence and 

impact on aerodynamic performance. For swept wings, the questions that arise have to do with the 

interaction of this flowfield with the spanwise and vortex flows.

Fortunately, in the case of roughness there is a fair amount of experimental data in the literature from 

studies unrelated to icing. The National Advisory Committee for Aeronautics (NACA) has conducted 

numerous experimental investigations into the aerodynamics of swept-wing configurations. Many of the 

studies explored Reynolds and Mach number effects and included cases with carborundum grains applied 

to leading edges. Neely and Conner (Ref. 7) investigated leading-edge roughness effects in the Langley 

19-ft pressure tunnel on a full-span wing with aspect ratio 4, 42° leading-edge sweep, mean aerodynamic 

chord = 34.71 in., and taper ratio = 0.625. Aerodynamic testing was conducted over a Reynolds number 

range of 1.7×106 to 9.5×106 with corresponding Mach number range of 0.10 to 0.22. Flow visualization 

showed that the stall of the clean wing began with separated flow near the trailing-edge of the outboard 

sections and progressed forward and inboard on the wing. In contrast, with k/cmac = 0.00032 diameter 

carborundum grains applied between x/c = 0.08 on the lower surface to x/c = 0.08 on the upper surface 

covering approximately 5 to 10 percent of the area, flow separation began at the leading-edge outboard 

sections. The effect of Reynolds number and surface roughness on the lift and drag is shown in Figure 1

and Figure 2, respectively. It can be seen that for all Reynolds numbers, the application of leading-edge 

roughness reduced CLmax, reduced the stalling angle of attack and increased the drag. In addition, the 

roughness reduced the effects of Reynolds number. All of these trends are typical of what has been 

observed in numerous airfoil studies with roughness. 

Kind and Lawrysyn (Ref. 8) numerically investigated the effects of wing frost on a jet transport wing 

with an aspect ratio of 7.9, 30° sweep and taper ratio of 0.25. Figure 3 shows the effects of roughness 

with k/cmac = 0.00006 on lift and drag for various chordwise extents of the roughness. The numbers in the 

figure legend indicate the chordwise extent of the roughness on the wing upper surface: “50” indicates the 

roughness extended from x/c = 0.42 to the trailing edge, “75” indicates x/c = 0.20 to the trailing edge, 

“95” indicates x/c = 0.009 to the trailing edge and “99” indicates x/c = 0.00 to the trailing edge. Their 

results indicate that the roughness reduced both the slope of the lift curve and CLmax for all levels of 

coverage, but the wing was most sensitive to roughness located in the first 1 percent of the chord.

Figure 1.—Effect of Reynolds number and leading-edge roughness on the lift coefficient of a 42° leading-edge swept 

wing, after Neely and Conner (Ref. 7).
NASA/TM—2013-216556 4

Figure 2.—Effect of Reynolds number and leading-edge roughness on the drag polar of a 42° leading-edge swept 

wing, after Neely and Conner (Ref. 7).

Figure 3.—Effect of simulated upper surface frost on a 

generic jet transport wing, after Kind and Lawrysyn (Ref. 8).
NASA/TM—2013-216556 5

Papadakis et al. (Ref. 9) performed an investigation of roughness on the leading edge of a business jet 

T-tail configuration. Sandpaper was used for the roughness with grit sizes of 40, 80, 120 and 180 

covering from approximately x/c = 0.025 on the lower surface to x/c = 0.016 on the upper surface. The 

chordwise extent was determined from a droplet trajectory analysis. The horizontal tail had a leadingedge sweep of 29°, aspect ratio 4.4 and taper ratio of 0.43. A full-scale model was tested in the NASA 

Ames 40- by 80-ft wind tunnel and a 25 percent scale model was tested in the Walter H. Beech 

Memorial 7- by 10-ft tunnel. The full-scale model had a mean aerodynamic chord of 49.2 in. and the 

25 percent scale model had a mean aerodynamic chord of 12.3 in. The 25 percent scale model was tested 

at a maximum Reynolds number of 1.36×106

, where it was found that the 40 and 80 grit roughness 

(k/cmac = 0.001375 and 0.000605) reduced CLmax by 4.5 and 2.8 percent respectively; however, the 120 and 

180 grit roughness (k/cmac = 0.000365 and 0.000259) increased CLmax by 4.9 and 1.9 percent, respectively.

Testing of the full-scale model at Re = 5.1×106 showed that the 40 and 120 grit roughness (k/cmac = 

0.000344 and 0.000091) decreased CLmax by 18.5 and 15.9 percent respectively. Surface pressure 

measurements and boundary layer profile measurements at x/c = 0.6 and 2y/b = 0.51 showed that the 

leading-edge roughness led to significantly thicker boundary layers and trailing-edge separation.

These results demonstrate the possible large aerodynamic penalties of roughness but also a potential 

problem of testing at low Reynolds numbers. Low Reynolds number experiments can mask the true 

penalties of ice shapes because the clean wing performance generally suffers as a result of the low 

Reynolds number more than the iced wing. In the 2-D case, it has been shown that the Reynolds number 

typically has a substantially smaller effect on the iced airfoil aerodynamics than for the clean airfoil

(Ref. 2). This is because the ice shape fixes the separation location thereby eliminating a potential 

mechanism through which the Reynolds number can affect performance. Therefore, while low Reynolds 

number experiments of iced airfoils are generally representative of higher Reynolds number, the low 

Reynolds number clean airfoil experiments may not and care must be taken when interpreting results. The 

low Reynolds number of the 25 percent scale experiments of Papadakis (Ref. 9) may explain the increase 

in CLmax for the 120 and 180 grit roughness, because the classification of roughness does have the 

potential to depend more on the Reynolds number than other classifications (Ref. 2). It should be noted 

that due to the lack of high Reynolds number swept wing icing experiments, it is unknown how the 

Reynolds number effects iced swept wings.

While these roughness studies do provide some initial insights, more research is needed to understand 

the effect of size, location, concentration, Reynolds number and other factors important to the 

aerodynamics of iced-swept wings. There is very little or no flowfield information such as surface 

pressures, flow visualization and velocity profiles in these reports. An understanding of the roughness 

flowfield is important to accurate subscale simulation of roughness effects.

2.2 Horn Ice

On airfoils, glaze icing conditions often lead to leading-edge ice accretion known as horn ice. There 

are several geometric features that are important to the horn-ice shape, including height, the angle it 

makes with respect to the flow and its surface location (Ref. 2). For a swept wing in glaze or mixed icing 

conditions, ice accretion can be divided into three subcategories known as complete scallops, incomplete 

scallops and no scallops. An illustration (Ref. 4) of each shape is shown in Figure 4, and photographs

(Ref. 10) of a complete and incomplete scallop ice accretion are shown in Figure 5. The type that forms 

has been found to be dependent on sweep angle and icing conditions (Ref. 4). Based on the aerodynamic 

classification system proposed here, the ice accretions in Figure 4 can be classified as horn ice. It will be 

shown in this section that on swept wings a further subclassification of “nominally 3-D” or “highly 3-D”

horn ice is necessary. These subclassifications are based on the fundamental aerodynamics of the 

particular ice shape which will be explained in this section. Based on available data, ice accretions with 

no scallops or incomplete scallops can be considered nominally 3-D horn ice while accretions with full 

scallops can be considered highly 3-D horn ice. 
NASA/TM—2013-216556 6

Figure 4.—Ice accretion on a swept wing in glaze icing conditions. Arrows indicate direction of 

flow. (a) Complete scallops, (b) Incomplete scallops, and (c) No scallops (Ref. 4).

Figure 5.—Photographs of complete and incomplete scallop glaze ice accreted 

on a wing having 28° leading-edge sweep in the NASA Glenn Icing 

Research Tunnel (Ref. 10).

In contrast to ice roughness on swept wings, there is some fundamental research regarding the 

flowfield about a 3-D wing with horn ice. An early experimental study by Khodadoust and Bragg

(Ref. 11) and an accompanying CFD study by Kwon and Sankar (Ref. 12) investigated the performance 

and flowfield of a semispan wing with a chord of 15-in., sweep angle of 30° and an aspect ratio of 

approximately 2.3. A NACA 0012 airfoil section was used. The ice shape used for the experiment and the 

CFD study was a simulation of a horn ice shape that was formed on a NACA 0012 airfoil in NASA’s 

IRT. Figure 6 shows the cross sections of the IRT generated ice shape and the simulated shape used for 

the experiment and the CFD study. The 2-D cross section was extruded along the span and in the 

framework of the proposed aerodynamic classification system this ice shape is nominally 3-D horn ice.

Figure 7 shows experimental pressure distributions at several spanwise locations with and without the 

horn ice at α = 8° and Re = 1.5×106

. The pressure distributions show that when the horn ice is present the 

leading-edge suction peaks are replaced by regions of nearly constant pressure covering approximately 20 

to 40 percent of the chord. This is very similar to the pressure distribution on an airfoil with a horn shape 
NASA/TM—2013-216556 7

where the pressure plateau indicates a separation bubble. In the case of a swept wing however, the 

separated flow and spanwise pressure gradient result in a 3-D leading-edge vortex. This vortex can clearly 

be seen in Figure 8 which shows surface oil flow and a particle trajectory simulation from the CFD results 

for the iced wing at the same conditions as Figure 7. The vortex grew in diameter moving outboard along 

the wing, and was lifted off the wing surface and shed into the wake near the tip. The effect of the 

increasing diameter of the vortex is seen in the broadening and reduction of the pressure peaks in Figure 

7. The simulated surface-oil flow showed regions of purely spanwise flow near the trailing edge and 

regions of almost completely reversed flow near the leading edge. These flowfield characteristics are 

analogous to the fundamental studies of swept wing stall conducted by Poll (Ref. 13).

Figure 6.—Measured and simulated 

glaze ice shape (Refs. 11 and 

12).

Figure 7.—Pressure distributions for the NACA 0012 swept wing with and without the horn ice shape, α = 8°, 

Re = 1.5×106

, after Bragg and Khodadoust (Ref. 11).

(a) Clean Wing (b) Wing with horn ice
NASA/TM—2013-216556 8

Bragg et al. (Ref. 14) used LDV to study the flowfield of the same swept NACA 0012 wing used by 

Khodadoust (Ref. 11) and Kwon (Ref. 12). The LDV was used to measure all three components of 

velocity at several spanwise locations for α = 8o and Re = 1.0×106

. Streamwise velocity measurements 

showed the inviscid flow accelerating over the ice shape leading to maximum values of u/U∞ of 1.53 and 

1.39 located at y/b = 0.4 and y/b = 0.7 respectively. Just downstream of the separation point, spanwise 

velocities of nearly twice the freestream velocity were measured. In general the velocities within vortex 

decreased as the tip was approached due to the increasing size of the vortex; however, near the tip large 

spanwise velocities due to the tip vortex were observed. Due to limitations of the experimental setup the 

only turbulence quantity obtained was u���′2�. Maximum turbulence intensities of nearly 35 percent were 

measured in the shear layer.

A more recent low-Reynolds number study by Diebold et al. (Ref. 15) investigated the performance 

and flowfield of a semispan wing with 35° leading-edge sweep, an aspect ratio of 8.3, taper ratio of 0.296 

and a mean aerodynamic chord of 6.9-in. A nominally 3-D horn ice shape was used. The ice shape 

simulation was formed by lofting several 2-D ice shape cross sections along the span. The model was 

based on the CRM, designed by Vassberg (Ref. 16), and was representative of a modern wide body 

commercial airliner. Experiments included force balance measurements, pressure sensitive paint, surface 

oil flow visualization and 5-hole probe wake surveys. Figure 9 shows oil flow images for the iced wing at 

several angles of attack for a Reynolds number of 3×105

. The oil flow clearly indicates the presence of a 

leading-edge vortex. In Figure 9, the reattachment line of the leading-edge vortex, which indicates where 

the separated shear layer has reattached to the surface, is highlighted. It can be seen that as the angle of 

attack increased, the reattachment line moved downstream indicating that the size of the leading-edge 

vortex grew. This is very similar to flowfield of an airfoil with a horn ice shape. As the angle of attack 

increases the separation bubble on the airfoil grows in the chordwise direction (Ref. 2). Pressure sensitive 

paint results on the swept wing indicated reduced pressure peaks and broad regions of nearly constant 

pressure under the vortex similar to Figure 7(b).

When an airfoil with horn ice reaches the stalling angle of attack, the separated flow fails to reattach. 

When a swept wing with horn ice stalls the flowfield is far more complex than for an airfoil. Figure 10

shows the oil flow of the stalled swept wing with ice. On the inboard sections of the wing the separated 

shear layer was able to reattach to the surface and the flow is qualitatively similar to the flow at lower 

angles of attack. Over the outboard sections of the wing however, the separated shear layer was unable to 

reattach and this section of the wing was stalled. The leading-edge vortex began near the root and quickly 

turned downstream and was shed into the wake creating a similar flowfield to that of the swept NACA 

0012 shown in Figure 8. 

Figure 8.—Flowfield over the NACA 0012 swept wing with horn ice shape: (a) CFD surface 

oil simulation and (b) CFD Particle trajectory simulation, α = 8°, Re = 1.5×106

, after Kwon 

and Sankar (Ref. 12).
NASA/TM—2013-216556 9

Figure 9.—Reattachment line of the separated flow on the iced wing for a range of angles. 

Re = 3x105

. Adapted from Diebold et al. (Ref. 15).

Figure 10.—Stalled iced wing with reattachment 

line highlighted. α = 6.5o, Re = 3x105

. Adapted 

from Diebold et al. (Ref. 15).

α = 2.36°

α = 3.36°

α = 4.40°

α = 5.46°
NASA/TM—2013-216556 10

Diebold and Bragg (Ref. 17) studied the performance of the same swept wing model as Diebold et al.

(Ref. 15) using a 5-hole probe wake survey analysis. Using methods described by Brune (Ref. 18), 

measurements of velocity and pressure in the wake were used to determine the lift and drag on the model, 

decompose drag into profile and induced drag components and measure the spanwise distribution of lift 

and drag. Figure 11 shows CL of the clean and iced wing plotted against the profile and induced drag 

components. The wake survey results show that the increase in drag due to the ice shape is exclusively the 

result of an increase in the profile drag. For the same CL, the ice shape does not affect the induced drag. 

The increase in profile drag due to the ice shape is a result of the large separated region behind the horn 

ice and the resulting pressure drag. In addition, the profile drag of the iced wing increased at much faster 

rate on the iced wing due to the increasing size of the leading-edge vortex which was seen in Figure 9. 

The wake survey results of Diebold and Bragg (Ref. 17) were also used to determine the spanwise 

distribution of lift and drag. Their results showed that for the particular ice shape and wing studied, the ice 

had a larger impact on the local aerodynamics of the outboard sections of the wing. This was explained as 

a result of a combination of the size of the ice shape relative to the local chord which increased as the tip 

was approached and the fundamental aerodynamics of swept wings. The spanwise flow on a swept wing 

acts as a form of boundary layer suction (Ref. 19) which may have helped promote reattachment of the 

separated flow. Diebold and Bragg (Ref. 17) were also able to relate features seen in the surface oil flow 

to features in the lift and drag distributions. It should be noted that the experiments of Diebold et al. and 

Diebold and Bragg were conducted at very low Reynolds numbers (Re < 1×106

) and it is unknown if the 

observations discussed here will hold at higher Reynolds number. 

Figure 11.—CL versus components of drag for clean and iced 

wing measured from wake survey. Re = 6x105 (Ref. 17).

CDp, CDi

CL

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9
NASA/TM—2013-216556 11

The studies discussed above provided valuable insight into the flowfield of a swept wing with a horn 

ice shape. To better understand the performance effects of horn ice on swept wings it is necessary to 

utilize different and more realistic ice shapes. Papadakis et al. (Ref. 20) investigated the effect of horn ice 

shapes on the T-Tail used in the roughness study (Ref. 9) discussed in Section 2.1. The model was a 

25 percent scale business jet T-Tail with a 29° leading-edge sweep, a mean aerodynamic chord of 

12.31 in. and an aspect ratio of 4.4. The ice shapes tested were generated using LEWICE (LEWICE

(Ref. 21) is a computational tool that simulates ice growth on surfaces exposed to icing conditions.), and 

in addition two spoilers consisting of a spanwise flat plate protruding normal to the airfoil surface were 

also tested. Cross-sectional views of the ice shapes and spoilers are shown in Figure 12. The ice-shape 

simulations were generated using the same atmospheric conditions but different simulation times, 9 and 

22 min. The spoilers were used to represent only the height of the horn on the suction surface. Two types 

of spoilers were used, for one type the height matched the LEWICE shape along the span while the other 

type of spoiler had a constant height equal to the maximum height of the ice shape. Note, the suction 

surface is the lower surface. In addition, the LEWICE ice shapes were tested with and without 24-grit 

(k/cmac = 0.0024) roughness was added. 

At a Reynolds number of 1.36×106 the ice shapes had a significant effect on performance. The 9 and 

22 min LEWICE shapes reduced CLmax 10.8 and 24.5 percent respectively. The addition of 24 grit 

roughness reduced CLmax by approximately 10 percent more than the corresponding smooth shapes. The 

addition of the roughness also increased the drag relative to the smooth shape. The large effect of added 

roughness, not typically seen in past airfoil studies, may be due to the aerodynamics of a swept wing; 

however, another potential explanation could be the geometry of the LEWICE simulations. The horns of 

LEWICE shapes are significantly more rounded than those of typical ice accretion. Because the horns are 

fairly rounded the point of separation may not be fixed to a specific location on the horn. Therefore, the 

addition of roughness may affect the separation location, whereas on a sharper horn this point would be 

fixed regardless of the surface condition. Surface pressure distributions showed that the roughness 

generally resulted in a reduced suction peak and a flatter broader Cp distribution, and boundary layer 

measurements near midspan at x/c = 0.6 showed the application of roughness generally resulted in thicker 

boundary layers. Both of these observations support the idea that the roughness led to earlier separation 

on the ice shape.

Figure 12.—Ice-shape simulations for 25 percent scale business jet T-Tail used by Papadakis et al. (Ref. 20).
NASA/TM—2013-216556 12

(a) 9-min ice shape (b) 22-min ice shape

Figure 13.—Comparison of smooth and rough LEWICE shapes with constant and variable height flat plate spoilers 

for a) the 9-min ice shape and b) the 22-min ice shape. Re = 1.36x106

. Adapted from Papadakis et al. (Ref. 20).

When investigating the effects of ice on aerodynamic performance it is advantageous to use simple 

simulations for the ice shape. Several studies (Refs. 22 and 23), have shown that very simple geometries, 

such as a leading-edge “spoiler,” can reproduce the performance characteristics of a horn-ice shape over a 

large angle of attack range. A simple geometry representing the height, angle, and location of the ice 

horn, essentially generates an equivalent separation bubble on the airfoil and, hence, very similar 

performance results. Figure 13 compares the lift of the smooth and rough LEWICE shapes, discussed 

above, with the constant and variable height flat plate spoilers. The designations L9 and L22 refer to the 

small and large smooth LEWICE shapes, and L9B and L22B refer to the same shapes with the 24 grit 

roughness added. The designations SP47C, SP47V, SP94C and SP94V refer to the spoilers. SP47 

corresponds to the L9 ice shape and SP49 corresponds to the L22 ice shape. The C and V refer to constant 

height and variable height respectively. Not unsurprisingly the spoiler with constant height, SP47C and 

SP94C had the largest effect because its height was equal to the maximum height of the ice shape and was 

constant along the span. Interestingly, the variable height spoilers, which matched the ice shapes height 

along the span, agreed well with the rough LEWICE shapes. It is perhaps surprising that the variable 

height spoiler matched the rough LEWICE shape as opposed to the smooth. This could possibly be 

because the location of the plate corresponds more closely to the separation point on the rough ice shape 

as opposed to the smooth shape; however, looking at surface pressure distributions and boundary layer 

measurements there are a few substantial differences between the rough LEWICE shape and the spoiler 

indicating the close match in performance may have been fortuitous. For example, pressure measurements 

at an angle of attack of –5° showed poor agreement in both the magnitude of the suction peak and width 

of the separated region on the root and midspan sections. 

The studies discussed above utilized ice shape simulations without any scallop features which can all 

be classified as nominally 3-D horn ice. Studies by Papadakis et al. (Refs. 24 and 25) investigated the 

effects of several different ice shapes on a swept wing that included high-fidelity, 3-D ice-casting 

simulations incorporating various levels of incomplete and complete scallops. The swept-wing model had 

a GLC-305 airfoil section aligned in the streamwise direction with a 28° leading-edge sweep, 18.72-in.

mean aerodynamic chord, 60-in. semispan and an aspect ratio of 6.8. The ice shapes used for this study 
NASA/TM—2013-216556 13

were formed on the same model in the NASA Glenn Icing Research Tunnel (Ref. 10). The icing 

conditions were chosen to produce a wide range of ice shapes including complete and incomplete scallops 

of various sizes and a rime ice formation. Moldings of the IRT ice accretions were subsequently used to 

produce the ice castings that were used for the aerodynamic tests. The ice-casting simulations captured 

the fully 3-D variation of the ice accretion.

The ice accretions tested and the corresponding tunnel conditions are shown in Table I. Ice1 (IRTCS10) is an example of a complete scallop glaze ice accretion. This shape can be compared with Ice2 

(IRT-IS10) which is an example of an incomplete scallop. While the cross-sectional tracings look similar, 

the photographs reveal the large differences in the 3-D geometry. Ice3 (IRT-SC5) is a rime ice accretion 

and will be discussed in Section 2.3. Ice4 (IRT-CS2) was formed at identical conditions to Ice1(IRTCS10), but with a much shorter exposure time of 2 min. Therefore, this is a relatively small glaze ice 

accretion and it is unclear if true scallop features formed due to the short exposure time. Ice5 (IRT-CS22) 

was also formed at identical conditions to Ice1 (IRT-CS10), but with a longer exposure time of 22.5 min.

This longer duration led to the very larger complete scallop accretion shown in Table I. Finally, Ice6 

(IRT-IPSF22) had some characteristics of complete scallops that were not as fully developed as in 

Ice1(IRT-CS10) and Ice5 (IRT-CS22). 

TABLE I.—ICE ACCRETIONS AND CORRESPONDING ICING

CONDITIONS USED BY PAPADAKIS ET AL. (REF. 24)

In addition to the IRT ice accretion in Table I, seven additional ice shapes were generated using the

ice accretion code LEWICE 2.0. Since LEWICE 2.0 is a 2-D ice accretion code, the 3-D simulations used 

for the aerodynamic testing were composed of 2-D slices at several spanwise sections of the wing blended 
NASA/TM—2013-216556 14

together. The detailed procedure is discussed in the original reference (Ref. 24). The LEWICE shapes 

were formed at the same conditions shown in Table I, but the velocity and airfoil geometry used in the 

computations was the velocity normal to the leading edge. The LEWICE shapes did not contain any 

scallop features and as will be shown below can be classified as nominally 3-D horn ice. These are 

designated with the prefix “LS” in the following figures. The geometry did vary in the spanwise direction 

but only according to the number of slices used to loft the 3-D geometry. The effect of ice-shape 

roughness was also investigated by adding 36-grit size roughness (k/cmac = 0.0011) to the smooth 

LEWICE shapes.

Because the Papadakis et al. (Refs. 24 and 25) study utilized high-fidelity ice-casting simulations of 

the 3-D ice accretions, it is possible to make some observations about the attendant aerodynamic effects 

relative to nominally 3-D horn ice with minimal 3-D features. Figure 14 shows a comparison of the ice 

accretion cross-section and the corresponding LEWICE shape for Ice2 (IRT-IS10) both of which can be

classified as nominally 3-D horn ice. Overall, there is reasonable agreement in the tracings, particularly 

for station A. Figure 15 compares the lift and CP distributions for the IRT generated shape and the smooth 

and rough LEWICE shapes. The results here are very similar to what has been reported for horn ice 

simulations on airfoils (Ref. 22). The effect on lift coefficient is very similar for all three configurations 

up to the stall region where the 3-D ice casting configuration has slightly lower maximum lift and stalling 

angle. For horn ice shapes on airfoils, small differences in the horn height, location or angle can 

significantly affect CL,max and may explain the differences given the obvious differences in the cross 

sections shown in Figure 14. The smooth LEWICE (LS-IS10) shape had slightly higher CL,max than the 

LEWICE shape with roughness (LS-IS10), which is consistent with the observations for the business jet 

T-tail (Ref. 20) discussed above and for past research on airfoils.

Also included in Figure 15 is a comparison of surface pressure at the 50 percent semispan stations at 

α = 4° For all three configurations, there are fairly well defined regions of approximately constant 

pressure from x/c ≈ 0.0 to x/c ≈ 0.2 on the upper surface. This pressure signature is similar to that shown 

previously in Figure 7 due to the leading-edge vortex induced by the horn geometry. In their report, 

Papadakis et al. (Ref. 25) provide more pressure comparisons at several angles of attack and for two 

additional spanwise locations. In most of these plots, the general shape of the distributions for the Ice2 

(IRT-IS10) casting is similar to that for the LEWICE simulations. Thus indicating that for incomplete 

scallops, the large flowfield features (such as the leading-edge vortex) are similar to ice shape with no 

scallops described earlier in this section. Therefore, from an aerodynamic perspective, shapes with no 

scallops and incomplete scallops fall under the same classification of nominally 3-D horn ice.

Figure 14.—Comparison of IRT Ice2 (IRT-IS10) and LEWICE ice-shape cross sections on GLC-305 

swept wing, after Papadakis et al. (Ref. 24)
NASA/TM—2013-216556 15

Figure 15.—Aerodynamic effect of IRT Ice2 (IRT-IS10) and LEWICE ice-shape simulations on GLC-305 

swept wing at Re = 1.8×106

, after Papadakis et al. (Ref. 24)

Figure 16.—Comparison of IRT Ice1 (IRT-CS10) and LEWICE ice-shape cross sections on GLC-305 

swept wing, after Papadakis et al. (Ref. 24).

The data presented in Figure 14 and Figure 15 for the incomplete scallop configuration of Ice2 (IRTIS10) are contrasted with the data for the complete scallop configuration of Ice1 (IRT-CS10). The crosssection geometry and LEWICE results are depicted in Figure 16 where there are much larger differences 

in the two geometries relative to that shown in Figure 14. This is most likely a result of the much higher 

degree of three-dimensionality associated with the complete scallop condition. It is not expected that the 

LEWICE cross sections in Figure 16 should match since LEWICE does not generate 3-D results. Even 

the most sophisticated ice accretion simulation codes do not predict the level of three dimensionality 

associated with a compete scallop ice accretion. It is also very difficult to compare a 2-D tracing of a 

highly 3-D ice accretion as small changes in spanwise location can have a very large effect on the crosssectional tracing.
NASA/TM—2013-216556 16

Given these differences in geometry, it is interesting to compare the aerodynamic results in Figure 17.

For example, the main effect on lift coefficient is not observed until the stall region where the differences 

in CL,max among the three configurations is larger than for the incomplete scallop case in Figure 15. The 

selected pressure distribution shown in Figure 17 is typical of the others plotted by Papadakis et al.

(Refs. 24 and 25). In this case, there is a sharp contrast between the pressure signatures of the Ice1 (IRTCS10) casting and the LEWICE ice-shape simulations. The upper-surface pressure distribution of the 

LEWICE configurations are very similar to what has been shown previously in Figure 7 and Figure 15.

This is particularly true for the case with roughness, LR-CS10. But for the Ice1 (IRT-CS10) casting, there 

is a gradual pressure recovery region with peak values of suction pressure much less than for the 

LEWICE shapes. A similar trend was observed for Ice6 (IRT-IPSF22) which also had a highly 3-D ice 

accretion geometry typical of complete scallops. Since there are significant differences in cross-section 

geometries between the ice castings and the LEWICE simulations for Ice1 and Ice6 (not shown), better 

matching of the surface pressure distributions is not expected. What is of interest here is that the shape of 

the pressure distributions is different in the area immediately downstream of the ice shape. For the 

nominally 3-D LEWICE shapes, there is a fairly distinct region of approximately constant pressure that 

has been shown to correspond to the leading-edge vortex. For the highly 3-D geometry of the complete 

scallop ice casting simulation, this pressure region was not observed and the peak suction pressures were 

much lower. To the authors’ knowledge, the flowfield immediately downstream of the highly 3-D

geometry has not been characterized or reported in the technical literature. These pressure data suggest 

that there exists a significantly different flowfield than for the nominally 3-D horn ice shapes reported in 

other swept-wing icing studies. Papadakis et al. (Ref. 24) suggest that the gaps between the scallop peaks 

allow high pressure air from the front face of the ice shape to leak through to the low pressure region 

behind the ice shape and alter the pressure distribution. Since current state-of-the-art ice accretion 

simulation codes are unable to predict highly 3-D features such as complete scallops, it is important to 

understand their impact on the aerodynamics. More research is needed to understand the flowfield 

associated with highly 3-D horn ice on swept wings so that the resulting impacts on wing performance 

may be better explained.

Figure 17.—Aerodynamic effect of IRT Ice1 (IRT-CS10) and LEWICE ice-shape simulations on GLC-305 

swept wing at Re = 1.8×106

, after Papadakis et al. (Ref. 24).
NASA/TM—2013-216556 17

Unlike the situation for other ice accretion on swept wings, there are currently both experimental 

performance measurements as well as flowfield studies for horn ice; although these studies offer valuable 

insights into the effect of horn ice accretions on swept wings there is still a substantial lack of information 

relative to the airfoil case. For example, a shortcoming of the studies discussed in this section is that no 

attempt was made to ensure that the IRT generated ice shapes were accurate representations of full-scale 

ice accretion. Other areas where experimental data are needed include more flowfield studies for detailed 

ice shapes with and without scallops and different wing geometries, parametric studies investigating 

geometrical features of the ice such as height, shape (e.g., tip radius, roughness level), location and the 

influence of wing geometry. 

The data presented in this section for horn ice on swept wings suggest an aerodynamic 

subclassification that distinguishes between “nominally 3-D” horn ice and “highly 3-D” horn ice. 

Nominally 3-D horn ice is associated with glaze ice accretion having either no scallops or incomplete 

scallops. This is only nominally 3-D since the gross shape does not vary significantly over small spanwise 

distances. Highly 3-D horn ice is associated with glaze ice accretion having complete scallops. In this 

case, the ice geometry changes significantly over small spanwise distances. From the perspective of 

aerodynamic classification, the difference between nominally 3-D and highly 3-D horn ice is defined in 

terms of the flowfield characteristics. The results discussed in this section for Figure 7 to Figure 15 were 

all associated with nominally 3-D horn ice as there was very little change in the simulated ice geometry in 

the spanwise direction. The flowfield was described in terms of the leading-edge vortex and the resulting 

region of approximately constant surface pressure aft of the ice shape. This was contrasted against some 

results for highly 3-D horn ice in Figure 16 and Figure 17 where the pressure signatures exhibited more 

gradual pressure recovery in place of the constant pressure regions aft of the ice shape. Due to a lack of 

data for highly 3-D horn ice on a swept wing, the key flowfield characteristics in this case are unknown.

To further distinguish between nominally 3-D and highly 3-D horn ice flowfield characteristics is yet 

another area where further research is required.

2.3 Streamwise Ice

This section will briefly discuss the classification of streamwise ice which is most often associated 

with rime icing conditions (Ref. 2). They typically follow the wing leading edge contour or form a hornlike shape, or protuberance, oriented into the flow direction. The available literature on this classification

is very sparse. Papadakis et al. (Ref. 24) tested one ice shape that could be classified as streamwise, Ice3 

in Table I. Figure 18 shows the effect of the IRT generated streamwise ice (IRT-SC5) as well as the 

smooth and rough LEWICE shapes. Although not shown here, for this particular case LEWICE predicted 

the ice shape very well. It can be seen that the IRT shape as well as both LEWICE shapes increased CLmax. 

An increase in the maximum lift due to a streamwise ice shape has been observed on airfoils (Ref. 26)

and has been attributed to the ice shape effectively forming a leading-edge flap with the increase in chord 

length and wing area relative to the reference area. Another potential factor may be the low Reynolds 

number at which the experiments were performed.

There is clearly a need for more aerodynamic data for streamwise ice on swept wings. It is expected 

that streamwise ice will not improve maximum lift performance in most cases. More information about 

the flowfield is required to understand the effects of roughness on streamwise ice that are thought to be

important to the aerodynamics. For streamwise ice on airfoils, the aerodynamic effects were chiefly made 

manifest through trailing-edge separations. While small leading-edge separation bubbles were often 

observed at the ice/airfoil juncture this flowfield feature did not play a decisive role in the resulting 

aerodynamics. For swept wings, it is expected that this separation, if present, may lead to leading-edge 

vortex formation. It remains to be determined which of these effects, or perhaps both, contribute to the 

observed performance changes on the iced swept wing.
NASA/TM—2013-216556 18

Figure 18.—Effect of streamwise ice on lift. 

Re = 1.8x106

. Adapted from Papadakis et al. 

(Ref. 24).

2.4 2.4 Spanwise Ridge Ice

Spanwise-ridge ice can be associated with a number of icing conditions where the wing leading edge 

is free of ice with sometimes large ice formations located farther downstream. Typical examples are SLD 

icing conditions coupled with ice-protection system operation. Large drops can impinge on the wing aft of 

the protected areas sometimes forming an ice accretion best described as a ridge. Spanwise-ridge ice can 

also form when a heated leading-edge, ice-protected surface is not evaporating all of the impinging water.

The liquid water flows downstream from the ice-protection system where it freezes forming a ridge 

oriented in the spanwise direction.

Like streamwise ice, there is very little available data on spanwise ridge ice. Papadakis et al. (Ref. 27)

performed a parametric study of spanwise-ridge ice on the same, swept GLC-305 model used by 

Papadakis et al. (Ref. 24). Ice shapes were simulated by uniformly extending the simple geometries, 

shown in Table II, across the span of the wing. The method of simulating spanwise-ridge ice with simple 

geometries is common practice for airfoils (Ref. 28). It is unknown if any effort was made to use 

simulations that accurately represented any documented full-scale ice accretion. The ridge heights ranged 

from 0.2 to 0.5-in. corresponding to 1 to 2.7 percent of the 18.72-in. mean aerodynamic chord. Each iceshape simulation was tested at 2.5, 5, 10, 15, 20 and 30 percent chord measured in the streamwise 

direction. The effect of ridge size could be assessed by comparing performance measurements for the ice 

simulation RB-2 and RB-6 which showed that for a given chordwise location the larger ridge had a more 

significant impact. The importance of ridge shape could be observed by comparing results for RB-4 and 

RB-5 which showed that RB-4, with the flat surface facing forward, resulted in lower CLmax and higher 

drag than RB-5, with the round surface facing forward. Similar results were observed in parametric 

studies of spanwise ridge ice on airfoils (Ref. 29) These parametric studies with airfoils showed that the 

chordwise location of the ridge was very important, and that the most severe penalties occurred when the 

ridge was located in a region of strong adverse pressure gradient (Ref. 2 and 29). In their test, Papadakis 

et al. (Ref. 27) observed that a given spanwise ridge simulation had the largest effect when it was located 

at 2.5 percent of the chord. Figure 19 shows the effect of all six simulated spanwise ridge shapes on the 

pressure distribution at 15 percent semispan of the wing for two different locations of the ice shapes, 2.5

and 15 percent. The angle of attack was 4° and the Reynolds number 1.8×106

. It can be seen that the 

suction peak of the clean wing was located at approximately 1 percent of the chord and therefore the 

spanwise ridge at 2.5 percent was located in a region of severe adverse pressure gradient. Figure 19(a)

shows that when the ice shape was located at 2.5 percent it prevented the formation of the initial suction 
NASA/TM—2013-216556 19

peak. The suction peak observed in the figure is due to the flow accelerating over the ice shape rather than 

around the leading edge. In contrast, Figure 19(b) shows that when the ice shape was located at 15 percent

the leading-edge suction peak was able to form which resulted in increased lift. Very similar results were 

observed by Lee and Bragg (Ref. 29) during their experiments with spanwise ridges on airfoils.

TABLE II.—SIMULATIONS OF SPANWISE RIDGE

ICE USED BY PAPADAKIS ET AL. (REF. 27)

(a) Ice shape at x/c = 0.025 (b) Ice shape at x/c = 0.15

Figure 19.—Effect of different spanwise ridge ice simulations on pressure distribution at 15 percent span of GLC-305 

swept wing at α = 4° and Re = 1.8×106. Ice shape located at (a) 2.5 percent and (b) 15 percent chord. Adapted 

from Papadakis et al. (Ref. 27).
NASA/TM—2013-216556 20

All of the ice shapes tested by Papadakis et al. (Ref. 27) resulted in reduced lift curve slopes and 

increased drag regardless of the chordwise location; however, for most of the ice shapes and locations 

tested the maximum lift coefficient increased relative to the clean wing. The effects of the simulated ice 

shapes, located at 15 percent chord, on lift are shown in Figure 20. It can be seen that although the lift 

curve slope was reduced for the iced cases, the stalling angle of attack and maximum lift coefficient 

increased.

Increases in lift coefficient at high angles of attack have been observed for spanwise-ridge ice 

accretions on airfoils. Whalen (Ref. 30) and Papadakis et al. (Ref. 31) investigated the effects of 

simulated spanwise-ridge ice accretions on airfoils at Reynolds numbers of 1.8×106 and 2.0×106

, 

respectively. They both observed that when the clean airfoil exhibited trailing-edge stall and the height of 

the ice simulation was comparable to the local boundary-layer thickness, the iced airfoil performed better 

at angles of attack near and above clean wing stall. This performance enhancement was attributed to the 

mixing layer generated by the ice shape entraining higher-momentum fluid into the boundary layer. This 

explanation may not suffice for the case of the ice simulations shown in Table II. The size of these 

simulations ranged from 0.2-to 0.5-in., likely making them significantly larger than the local boundary 

layer. It is also important to note that Broeren et al. (Ref. 32) showed that this lift enhancing effect on 

airfoils can be an artifact of low-Reynolds number testing of the clean airfoil. Their results showed that it 

is possible for the iced airfoil to have better high angle of attack performance characteristics than the 

clean airfoil at a Reynolds number of 1.8×106

, but when compared to the clean airfoil at a Reynolds 

number of 15.9×106 the iced airfoil performance degraded substantially. Due to these results it is difficult 

to determine the exact cause of the performance enhancement seen by Papadakis et al. (Ref. 27). This also 

emphasizes the importance of investigating Reynolds number effects where such data are lacking.

Future research on spanwise ridge ice should explore Reynolds number effects. The authors are 

unaware of any swept-wing studies that used high-fidelity spanwise ridge ice shapes formed from castings 

of actual ice accretions. As a result, there are no data that can be used to validate the method of simulating 

ice shapes with simple geometries on swept wings. It is well known that real spanwise-ridge type ice 

accretion can be highly 3-D with significant spanwise variation in the gross shape. Any future work must 

be supplemented with extensive flowfield studies in order to improve our understanding of how the ice 

accretions affect the aerodynamics of swept wings. Of particular interest is the spanwise vortex 

interaction with the spanwise ridge in determining the attendant aerodynamic effects.

Figure 20.—Effect of simulated spanwise-ridge ice 

shapes located at 15 percent chord on GLC-305 

swept wing, Re = 1.8×106

, after Papadakis et al.

(Ref. 27).
NASA/TM—2013-216556 21

3.0 Summary

The continued design, certification and safe operation of swept-wing airplanes in icing conditions rely 

on the advancement of computational and experimental simulation methods for higher fidelity results 

over an increasing range of aircraft configurations and performance, and icing conditions. There is 

increasing demand to balance trades-offs in aircraft efficiency, cost and noise that tend to compete 

directly with allowable performance degradations over an increasing range of icing conditions. Aircraft 

icing research has now reached the level of maturity that computational methods and experimental tools 

are currently being used to address many of these challenges. However, knowledge gaps do remain for 

swept-wing geometries and larger droplet icing conditions. The current state-of-the-art in icing 

aerodynamics is mainly built upon a comprehensive understanding of 2-D geometries developed from 

myriads of research efforts described in the technical literature. Such an understanding for fundamentally 

3-D geometries such as swept wings does not currently exist. The purpose of this report is to describe 

what is known of iced-swept-wing aerodynamics; to identify the type of research that is required to 

improve the current understanding; and to develop an aerodynamically based classification of swept-wing 

ice accretion. This report focuses on the fundamental aerodynamics of iced swept wings. The existing 

data tend to be: (1) mostly at low-Reynolds number and (2) applicable to simple swept-wing geometries 

that do not have high-lift systems, wing-mounted engines, fuselages and other features of actual airplane 

wings. These factors can significantly alter the iced aerodynamics for particular configurations and so 

extreme caution must be exercised in terms of making general conclusions based upon the current, limited 

database.

Ice accretion formations on swept wings can have unique characteristics. Depending upon specific 

icing conditions and sweep angle, the region of the attachment line may not be smooth as is often the case 

for airfoils. While initial roughness and rime ice accretion on swept wings tend to look very similar to that 

on airfoils, there can be significant differences for glaze ice accretion. For glaze icing, certain 

combinations of icing conditions and sweep angle can lead to the formation of highly 3-D features called 

scallops that do not exist for ice accretions on airfoils. It is also possible to have glaze ice accretion with 

no scallops or even incomplete scallop formations on swept wings.

Following the method used in a previous review of iced-airfoil aerodynamics, this report classifies 

swept-wing ice accretion into four groups that are based upon unique flowfield features. Instead of relying 

upon ice accretion terminology such as rime and glaze, the four aerodynamic groups have names 

associated with ice-shape geometry. These four groups are: ice roughness, horn ice, streamwise ice and 

spanwise-ridge ice. This report attempts to describe the unique flowfield features of each group that 

determines the iced-wing aerodynamics:

• Ice roughness represents initial leading-edge ice accretion and a key aerodynamic characteristic is 

that the scale of the boundary-layer separation is of the same order as the size of the roughness. 

While there are many studies that have looked at roughness effects on swept-wing performance, 

including Reynolds number effects, there is a lack of flowfield data from which to interpret these 

results. More data are needed to understand the effects of roughness size, location and 

concentration on swept-wing aerodynamics.

• Horn ice is large, leading-edge ice accretion that can be associated with glaze icing conditions. 

The flowfield is characterized by large-scale, boundary-layer separation originating at the horn.

This separation leads to the formation of a spanwise-running, leading-edge vortex that is similar 

to that found on clean swept wings with leading-edge separation. There are a number of lowReynolds number studies that have characterized the horn-ice flowfield for swept wings and 

documented the behavior of the leading-edge vortex preceding wing stall. This presents an 

excellent starting point, especially for nominally 3-D horn shapes such as those with no scallops 

or even incomplete scallops. However, there are no flowfield data known to the authors for highly 
NASA/TM—2013-216556 22

3-D horn ice such as complete scallop formations. Therefore, the fundamental aerodynamics are 

essentially unknown in this case. This is an important factor since the associated performance 

penalties may be large. The small amount of existing data indicate that there are fundamental 

flowfield differences between nominally 3-D horn ice characterized by no scallop formations 

versus highly 3-D horn ice characterized by fully developed scallop formations. Flowfield data, 

such as mean and fluctuating velocity profiles and surface shear stress are needed to further 

understand the important differences observed between these two cases of horn ice on swept 

wings.

• Streamwise ice can be associated with rime icing conditions and is generally conformal to the 

wing leading edge, or may form a horn-like feature (or protuberance) oriented into the flow 

direction. The only example of this group cited in this report showed an increase in wing 

maximum lift coefficient with the streamwise ice. While this effect may be possible, it is not 

expected to hold for most cases and illustrates the need for further wing performance data and 

flowfield information with realistic streamwise ice simulations.

• Spanwise-ridge ice can be associated with ice protection system operation in SLD icing 

conditions or incomplete evaporation of impinging water. The leading edge is free of ice with an 

ice ridge located downstream often in the range of 10 to 15 percent chord. This report describes 

data from only one low-Reynolds number study for very simple geometric representations of 

spanwise-ridge ice on a swept wing. More aerodynamic performance data and flowfield 

information are needed for realistic spanwise-ridge ice simulations.

For all of the proposed ice-shape classifications, relatively little is known about the 3-D flowfield and 

even less about the effect of Reynolds number and Mach number on these flowfields. Both of these 

deficiencies are important and limit the ability to classify swept-wing ice accretion. Most of the data 

found in the literature pertain only to aerodynamic performance. Except for nominally 3-D horn ice, 

flowfield information is limited to some pressure distributions, all at low-Reynolds number. Variations in 

Reynolds number found for iced-swept wings are all for relatively low-Reynolds number and provide no 

guidance as to the appropriateness of these data at Reynolds numbers approaching flight. In the 2-D case, 

Reynolds and Mach number effects have been shown to be small in most cases and low-Reynolds number 

data have been used extensively to classify ice shapes and improve our understanding of iced-airfoil 

flowfields and aerodynamics. The very limited data available on swept wings to date suggest a similar 

result, but much more data are needed, particularly for realistic ice-shape simulations at higher Reynolds 

numbers.

The classifications and supporting data presented in this report can serve as a starting point as new 

research explores swept-wing aerodynamics with ice shapes. As further results become available, it is 

expected that these classifications will need to be revised just as has occurred in the airfoil case.

References

1. Lynch, F.T., and Khodadoust, A., “Effects of Ice Accretions on Aircraft Aerodynamics,” Progress in 

Aerospace Sciences, Vol. 37, No. 8, Nov. 2001, pp. 669-767.

2. Bragg, M.B., Broeren, A.P., and Blumenthal, L.A., “Iced-Airfoil Aerodynamics,” Progress in 

Aerospace Sciences, Vol. 41, No. 5, Jul. 2005, pp. 323-362.

3. Broeren, A.P., Diebold, J.M. and Bragg, M.B., “Aerodynamic Classification of Swept-Wing Ice 

Accretion,” NASA/TM—2013-216381, 2013

4. Vargas, M., “Current Experimental Basis for Modeling Ice Accretions on Swept Wings,” Journal of 

Aircraft, Vol. 44, No. 1, Jan.-Feb. 2007, pp. 274-290.

5. Anderson D. N. and Shin J., “Characterization of Ice Roughness from Simulated Icing Encounters,” 

AIAA Paper 1997-0052.

6. Shin, J., “Characteristics of Surface Roughness Associated with Leading-Edge Ice Accretion,” AIAA 

Paper 1994-0799.
NASA/TM—2013-216556 23

7. Neely, R.H, and Connor, D.W., “Aerodynamic Characteristics of a 42o Swept-Back Wing With 

Aspect Ratio 4 and NACA 641-112 Airfoil Sections at Reynolds Numbers From 1,700,000 to 

9,500,000,” NACA RM L7D14, 1947.

8. Kind, R.J., and Lawrysyn, M.A., “Effects of Frost on Wing Aerodynamics and Take-Off 

Performance,” Paper 8 in “Effects of Adverse Weather on Aerodynamics,” AGARD-CP-496, Dec. 

1991.

9. Papadakis, M., Yeong, H.W., Chandrasekharan, R., Hinson, M., and Ratvasky, T.P., “Effects of 

Roughness on the Aerodynamic Performance of a Business Jet Tail,” AIAA Paper 2002-0242, Jan. 

2002.

10. Vargas, M., Papadakis, M., Potapczuk, M.G., Addy, H.E. Jr., Sheldon, D., and Giriunas, J., “Ice 

Accretions on a Swept GLC-305 Airfoil.” SAE 02GAA-43, SAE General Aviation Technology 

Conference and Exhibition, Wichita, KS, Apr. 2002, Also NASA/TM—2002-211557, Apr. 2002.

11. Khodadoust, A. and Bragg, M.B., “Measured Aerodynamic Performance of a Swept Wing with a 

Simulated Ice Accretion,” AIAA Paper 1990-0490.

12. Kwon, O.J., and Sankar, L.N., “Numerical Study of the Effects of Icing on Fixed and Rotary Wing 

Performance,” AIAA Paper 91-0662, Jan. 1990.

13. Poll, D.I.A., “Spiral Vortex Flow Over a Swept-Back Wing,” Aeronautical Journal, May 1986.

14. Bragg, M.B., Kerho, M.F., and Khodadoust, A., “LDV Flowfield Measurements on a Straight and 

Swept Wing with a Simulated Ice Accretion,” AIAA Paper 93-0300, Jan. 1993.

15. Diebold, J.M, Monastero, M.C. and Bragg, M.B., “Aerodynamic of a Swept Wing with Ice Accretion 

at Low Reynolds Number,” AIAA 2012-2795

16. Vassberg, J.C., DeHaan, M.A., Rivers, S.M. and Wahls, R.A., “Development of a Common Research 

Model for Applied CFD Validation Studies,” AIAA 2008-6929

17. Diebold J.M. and Bragg, M.B., “Study of a Swept Wing with Leading-Edge Ice Using A Wake 

Survey Technique,” To be presented at the 51st AIAA Aerospace Sciences Meeting, January 2013.

18. Brune, G. W., “Quantitative Low Speed Wake Surveys,” Journal of Aircraft Vol. 31, No. 2, 1994.

19. Hoerner, S.F., Fluid-Dynamic Lift, Hoerner Fluid Dynamics, Brick Town, NJ, 1975.

20. Papadakis, M., Alansatan, S., and Yeong, H.W., “Aerodynamic Performance of a T-Tail with 

Simulated Ice Accretions,” AIAA Paper 2000-0363, Jan. 2003.

21. Wright, W.B., “User’s Manual for LEWICE Version 3.2,” NASA/CR—2008-214255, Jan. 2008.

22. Busch, G.T., Broeren, A.P., and Bragg, M.B., “Aerodynamic Simulation of a Horn-Ice Accretion on a 

Subscale Model,” AIAA Paper 2007-0087, Jan. 2007.

23. Papadakis, M., Gile-Laflin, B.E., Youssef, G.M., Ratvasky, T.P., “Aerodynamic Scaling Experiments 

With Simulated Ice Accretions,” AIAA Paper 2001-0833, Jan. 2001.

24. Papadakis, M., Yeong, H.W., Wong, S.C., Vargas, M., and Potapczuk, M.G., “Aerodynamic 

Performance of a Swept Wing with Ice Accretions,” AIAA Paper 2003-0731, Jan. 2003.

25. Papadakis, M., Yeong, H.W., Wong, S, Vargas, M and Potapczuk, M., “Experimental Investigation of 

Ice Accretion Effects on a Swept Wing,” DOT/FAA/AR-05/39, Aug. 2005.

26. Bragg, M.B., and Gregorek, G.M., “Wind Tunnel Investigation of Airfoil Performance Degradation 

Due to Icing,” AIAA Paper 82-0582, Jan. 1982.

27. Papadakis, M., Yeong, H.W., and Wong, S.C., “Aerodynamic Performance of a Swept Wing with 

Simulated Ice Shapes,” AIAA Paper 2004-0734, Jan. 2004.

28. Busch, G.T, “Experimental Study of Full-Scale Iced-Airfoil Aerodynamic Performance Using SubScale Simulations,” Ph.D. Dissertation, Dept. of Aeronautical and Astronautical Engineering, Univ. 

of Illinois Urbana-Champaign, Urbana, IL, 2009. 

29. Lee, S. and Bragg, M.B., “Effects of Simulated-Spanwise Ice Shapes on Airfoils: Experimental 

Investigation,” AIAA Paper 99-0092, 1999

30. Whalen, E.A. “Aerodynamics of Runback Ice Accretions,” Ph.D. Dissertation, Dept. of Aeronautical 

and Astronautical Engineering, Univ. of Illinois Urbana-Champaign, Urbana, IL, 2007.

31. Papadakis, M., and Gile-Laflin, B.E., “Aerodynamic Performance of a Tail Section with Simulated 

Ice Shapes and Roughness,” AIAA Paper 2001-0539, Jan. 2001.
NASA/TM—2013-216556 24

32. Broeren, A.P., Whalen, E. A., Busch, G.T., and Bragg, M.B., “Aerodynamic Simulation of Runback 

Ice Accretion,” AIAA Paper 2009-4261, Jun. 2009.